Measure The Diameter of the Exhaust Throat - What's Your Problem?

The 90 Percent RuleMike Hamilton; Grimsby, Ontario, Canada: Of all the mags I'm writing to, you give the best tech answers-period. I've got a 350 Chevy in an S-15 truck with 1 1/2-inch headers, a 750-cfm Holley, a 300-degree-duration cam with 0.488-inch lift, a Weiand dual-plane intake, 1.94-inch heads, and 10.25:1 compression. I ran 13.2 at 100 mph with a 3,000-rpm stall converter and 4.11:1 gears. It ran 1.8 short (60-foot) times, jetted and tuned to get the max out of it. After a few years believing my 1.94 heads were the weak point, I wanted to go faster so I tried a set of 1970 441 castings with 2.02 and 1.60 valves. With some jetting, the truck only ran 1 mph faster!

2/8Use a snap gauge or a dial caliper to measure the inside diameter of the exhaust throat. For a 1.60-inch valve, the 90 percent rule means the throat should be no larger than 1.44 inches. Carefully blend the throat into the valve seat, and don't hit the seat with the grinder!

Previously, I overcammed the motor to prop up the 1.94 heads. If the cam is too big I can understand the same e.t., but why not more mph? Surely with 5,800 rpm at the top end it would be breathing right. I was hoping for at least 25 more horsepower, a few tenths, and more mph. What do you think?

Jeff Smith: I think the truck is running exactly as it should, Mike-it's just your expectations are too high. There really isn't an appreciable airflow difference between those two sets of heads. While you might think larger valves would improve the flow, there's more to the story than just big valves. The real cork with small-block heads is actually on the exhaust side. You mentioned that you crutched the engine with a bigger cam. But with the restricted exhaust, adding intake duration won't help. You didn't mention the specific cam timing, but the trick is to add about 8 to 10 degrees more exhaust duration than intake-such as a 230/240-degree dual pattern camshaft. This will extend the potential horsepower peak slightly, but even that trick is really just a crutch. The truth is that production iron small-block Chevy exhaust ports are weak. If you can't evacuate the exhaust gas from the chamber at high rpm, there will be exhaust gas that will remain in the chamber for the next power stroke. This exhaust gas won't burn again, so it reduces the cylinder pressure and the peak horsepower suffers. With a more efficient exhaust port, you can gain that 25 hp you were looking for. There are two ways to accomplish that.

The first is the least expensive, but it requires some effort and skill. Remove and disassemble the heads and have your machine shop evaluate both the intake and exhaust valveguides. If the guides are loose, you must rebuild them. You should also consider new valves, although I wouldn't go over 2.02/1.60 inches. Now comes the fun part. The place where you can make the most gain for the least amount of time involves concentrating on improving exhaust port flow. The effort will require considerable restraint on your part. If you look closely at the exhaust valve seat in the head, you will see a small ridge just under the seat. This area is called the throat. The idea is to eliminate this ridge while still leaving a slight venturi effect. The best way to accomplish this is to use what cylinder head porters call the 90 percent rule. Using the diameter of the exhaust valve as a reference, multiply the diameter by 90 percent: 1.60 x 0.90 = 1.44 inches. This will be the maximum diameter of the venturi area directly below the exhaust valve seat. Maintaining the throat diameter at 1.44 inches or smaller will maximize the overall flow potential of your pocket porting. This is not as easy as it might sound because it will be very tempting to hog out the throat far beyond the 90 percent point.

Know that you can increase max lift airflow by exceeding this 90 percent diameter rule. If all you were interested in was peak flow cfm at maximum valve lift-more than 0.600-inch valve lift-then you could increase the throat diameter by more than 90 percent. The problem is that this move is guaranteed to hurt the flow at the lower valve lifts such as the 0.200- through the 0.500-inch range. Several years ago, we comparison-tested of a set of Patriot aluminum heads against a set of factory iron Vortec heads. The Patriots offered an outstanding peak valve lift flow number, which was achieved with a larger-than-90-percent throat diameter on the intake side. On the engine dyno, the Patriot heads did make slightly more horsepower than the Vortecs. The stock iron Vortec heads, however, made far more torque in the midrange. The point of this is that if you improve the overall flow in the 0.200- to 0.300- and 0.400-inch valve lift areas, especially on the exhaust side, the engine will make more torque and more peak horsepower. That's where you will pick up that 25 hp at the peak, plus the engine will make more torque across the entire rpm band and be much more fun to drive. It's a win-win deal.

The second way to gain the extra power is the easiest but hardest on your wallet. The small-block Chevy enjoys the largest selection of cylinder heads on the planet, so there's no lack of heads from which to choose. The aforementioned Vortec iron heads are very inexpensive and flow well, although they too can benefit from pocket porting the exhaust. Find a set of those heads and do the exhaust port work, and they will be far better than a set of 441 heads. Next in line might be the Edelbrock E-Street small-block head (PN 60739, $1,093 from Summit Racing). It is an affordable aluminum head with 70cc combustion chambers and excellent flow numbers. The beauty of this head is that it offers substantially better flow numbers over the typical 441 iron head with 185cc intake ports and 2.02/1.60-inch valves, not to mention the weight savings of aluminum. Considering that completely rebuilding and modifying a set of iron factory heads might easily cost $700 or more, the Edelbrock heads are a great deal at only another $400. This would be my choice rather than going through all that work to make an old iron head work-unless you prefer swimming upstream. It certainly builds character!

More Info
Edelbrock
Torrance, CA
310/781-2222
Edelbrock.com

3/8<strong>CC Quickies</strong><br />
There was a British television show that aired in the U.S. back in 1967 that starred Patrick McGoohan as an ex-British secret agent who abruptly quit his profession and was interred in a picturesque seaside village. He was prevented from leaving by a sinister white balloon called The Rover that had the power to incapacitate or even kill. We now know where that balloon found a new home-right across from our El Toro suspension test site.

GM Performance Parts
Warren, MI
800/577-6888
gmperformanceparts.com

Summit Racing Equipment
Akron, OH
800/ 230-3030
SummitRacing.com

Knock ControlMichael Latine, via CarCraft.com: I have an '87 Trans Am that still has the original carbureted 305 and a five-speed. I am working on replacing it with a 406ci small-block and a T-10 four-speed. I would like to keep the computer-controlled Quadrajet and the computer control of the spark and knock sensor.

4/8The SafeGuard unit not only pulls the timing back before audible detonation occurs, but as soon as the detonation is gone, it will reintroduce the timing one step every 20 engine revolutions. It also has a boost retard function. List price is $595.

Where can I find a chip? None of the chip companies services my year car anymore. Will my stock ECM still work because of the mild nature (low rpm) range of this motor?

The 0.030-over 406 has cast pistons with 9.3:1 compression, 64cc Vortec iron heads, an Edelbrock Performer manifold, and a cam with 230 degrees of duration at 0.050-inch lift, with 0.455-inch lift and a 114-degree lobe-separation angle. It also has 1 5/8-inch block hugger headers with 2 1/2-inch collectors and exhaust. Peak torque is expected to be 3,600 rpm and peak horsepower is 5,100. The T-10 is a close ratio, and the rearend gears will be 3.27:1 with a BorgWarner 9-bolt posi.

The basis for my engine is a buildup on a similar 406 in Chevy High Performance in the June '03 issue written by Scott Crouse. I worked for Jim Russell Racing Drivers School at Laguna Seca (Infineon Raceway) running the garage and also taught the driving school toward the end of that time. I love cars and your magazines and have been a car nut for 55 years now!

Jeff Smith: I like your plan, Michael but I have a slightly different suggestion to approach the same level of control. Adapting the feedback Q-jet and the factory ECM to work with this much larger-cubic-inch engine would not be impossible, but it would certainly be problematic because it would require several attempts at part-throttle metering and timing, each requiring a new chip because your ECM predates the EE-prom erasable chips that late-model computers enjoy. I like the idea of the Q-jet and retaining the factory knock control. Keep in mind this won't be emissions legal, but it will work very efficiently.

Let's deal with the knock control first. A company called J&S Electronics sells a very slick electronic knock control device called the SafeGuard. It uses a factory-style detonation sensor that with J&S' electronic software efforts can identify the individual cylinder that rattled and retard the timing on that cylinder before it fires again allowing the other cylinders to continue with their normal timing settings and power levels. If the cylinder continues to detonate, the SafeGuard unit will pull up to 20 degrees out of that cylinder to prevent further detonation. We put this unit into action on a supercharged small-block Chevy on Ken Duttweiler's dyno and we were able to add timing to the engine until the SafeGuard began to set off the retard function on cylinders 5 and 7. Generally this happens around peak torque, where maximum cylinder pressure occurs. It's also possible to add a little more timing to make more power with the other cylinders while the one or two cylinders are retarded to prevent detonation damage. If you are interested, call John Pizzuto at J&S Electronics and talk over your application with him. In your case, you would disable the existing computer control of the ignition and use a stand-alone HEI or aftermarket ignition system to govern the spark curve. The SafeGuard system also includes a built-in rev control and a nitrous retard feature.

As for the carburetor side of things, I like the idea of using a standard, nonfeedback Q-jet for this application. The Quadrajet is an excellent street carburetor that continues to suffer from an undeserved poor reputation. I think the carb is ignored more because of its apparent complexity. The reality is that the combination of a primary metering rod inside the main jet produces a far more tunable combination if you take the time to come up with a lean, part-throttle tune-up. We would suggest investing in a quality air/fuel ratio meter such as the Innovate Motorsports unit or a similar piece to track your air/fuel tuning changes. It should be easy to get the main metering circuit to deliver 14.5:1 to even 15:1 at highway cruise at mild-throttle openings. Then you can adjust those huge secondaries to deliver 12.5:1 to around 13.0:1 air/fuel ratio at wide-open throttle (WOT) for best power. Pay particular attention to the idle feed restrictor size when choosing your carburetor. I'd suggest talking with a good Q-jet company such as Jet or Quick Fuel. They can dial in a carb with a relatively small idle-feed restrictor that is essential for not only a lean mixture at idle, but also for part-throttle cruise. If you want to be adventuresome, there is information in Doug Roe's Rochester Carburetors book on how to modify the idle circuit to produce the desired air/fuel ratio at low-throttle openings. It's a hassle, but well worth the effort.

It's All In The ReturnMichael Dalton, via CarCraft.com: I have an '82 Chevy shortbed truck with a 0.030-over 350 and a 268 Competition cam. I have had a complete TPI system for 20 years that I have been wanting to put in the truck. The only things missing are the computer and wiring harness, and I know where to purchase them. My biggest holdup is the fuel system and putting in a return line. Is there a company that makes a fuel-sending unit with a fuel pump and a return line that can be used for this application? I have been procrastinating for several years, but my 18-year-old son is pushing really hard to install this system. Any help you can give me will help get him off my back.

5/8This is a custom fuel pickup from the guys at RobbMc for a Chevelle, but the idea is to create a return back to the tank for fuel injection systems. The return concept also works very well for carbureted applications because the pump does not have to work as hard dead-heading into the regulator.

Jeff Smith: From looking at online photos of the stock fuel-sending unit for your truck, there is a vapor return line that looks like it would work perfectly as a return line back from the engine. The ideal situation with a return line is to minimize any restriction to flow, but with a standard 45-psi system, even a 1- to 2-psi return pressure is inconsequential. That means if you have plumbed a 3/8-inch-id feed line from the tank to an externally mounted fuel pump, and the same line up to the engine, I would suggest the same size line for the return to minimize the restrictions. Of course, this move will sacrifice the vapor canister return line feature on your truck, but in looking at photos of the replacement sending unit, there may be room to install your own separate return line. Or, you could easily use a cutoff wheel on a die grinder to eliminate the original 5/16-inch vapor return tube and drill the sender out to the outside diameter of 3/8-inch tubing and braze or silver-solder a new line into the existing sending unit. If you do this, take the time to add some length to the return line into the tank and make sure the end of the return is aimed 180 degrees away from the fuel tank pickup tube. This helps minimize fuel aeration in the tank. Ideally the inlet is placed as far away from the return line as possible, but a 6- to 8-inch separation is acceptable. That is how I built my original return line setup in my '65 Chevelle and that engine makes 550 hp that I use for road racing, so I don't think you'll have a problem.

Much is written about how to mount a fuel pump, and most stories claim the pump must be located below fuel level. While this is the ideal location for pump efficiency, it is possible to mount the pump either at or slightly above fuel level and still enjoy solid pump performance. That said, pumps do a far better job of pumping and pressurizing fuel than pulling fuel vertically out of the tank. So placing the pump below fuel level is a good idea because it allows gravity to assist by pushing fuel into the inlet side of the pump. Did you know that the Model A Ford placed the fuel tank on the firewall and gravity-fed fuel to the carburetor? It doesn't get any simpler than that.

6/8<strong>CC Quickies</strong><br />
Source Interlink just purchased a brand-new diesel Ford F-250 crew cab dualie that we broke in on the way up to Virginia City, Nevada. With up to 40 psi of boost, this baby towed our Chevelle like it weighed 500 pounds. Our scenic route return took us through Sonora Pass in the Stanislaw National Forest (where the signs told us a crew cab dualie towing a trailer shouldn't go!) and up over Monitor Pass at 8,000 feet! The towing power was awesome!

Mystery CamDominic O'Loughlin; Preston, Victoria, Australia: I love your column, it's the second most educational one in the magazine (next to Krass & Bernie, of course!). Can you please tell me the simplest and easiest way to determine unknown cam specs on my preloved V-8? All I know (and all the previous owners knew) is that it has a hydraulic cam and I've had it chassis-dyno-tested at 220 rwhp. The engine is a GM Holden V-8 originally 308 ci stroked to 355 inches. It is currently out of the car. I suspect the stock heads are holding back its true potential. I want to port them to suit the cam installed. Cheers from Australia!

7/8When we degree our small-block and big-block Chevy engines, I have a used lifter with the pushrod cup turned upside down. The center oil hole makes a great locating device for the dial indicator plunger.

Jeff Smith: It's great to hear that Car Craft has found its way Down Under, Dominic. While I don't know very much about the Holden V-8 engines, it really doesn't matter because the procedure for determining the cam timing with the cam in the engine is a relatively easy thing to do on any four-stroke engine. What you will need is a lifter, a degree wheel, a fabricated pointer, a piston stop, and a dial indicator to determine when the cam lobe opens and closes. The best way to do this is to access the lifters as opposed to attempting to read lifter movement off the pushrod.

The first thing to do is remove all the spark plugs and come up with a way to rotate the engine in both directions. The crank bolt may work, but what can often happen is the bolt loosens when turning the engine backward, the degree wheel spins, and you have to start all over again. Next, you will need enough room to mount a degree wheel on the front of the engine. That usually requires removing the crank pulleys to mount the degree wheel directly to the harmonic balancer along with a coat-hanger wire pointer. Rotate the engine by hand until No. 1 piston is roughly at top dead center (TDC). Line up the pointer to the TDC mark on the degree wheel. Now you'll need to turn the engine backward about half a turn and install a spark plug-style piston stop in the No. 1 spark plug hole. Slowly rotate the engine until the piston hits the piston stop and record the number of degrees on the wheel before TDC (BTDC). Next, rotate the engine backward until the piston hits the stop and record the degrees after top dead center (ATDC). To establish true TDC, the numbers need to be the same on either side of TDC at the stops. This step is absolutely essential to accurate cam readings.

Next mount the dial indicator plunger to indicate off the side of the lifter body as opposed to placing the dial indicator plunger rod in the pushrod cup. Because the plunger rod is smaller than the pushrod cup, it can slide around and produce inconsistent readings. If indicating off the lifter body is too difficult, you will need to fabricate some kind of centering device to keep the plunger located in the lifter. Now we're ready to start. This first procedure will read the cam timing from the intake lifter, which may not necessarily be the first lifter at the front of the engine. If you're not sure, line up the lifters with the cylinder head ports. Make sure the lifter freely moves up and down in its bore and that it always returns to zero lift during a full lift cycle. Rotate the engine until the lifter is fully seated and zero the dial indicator. Now rotate the engine clockwise and watch for 0.050 inch of lifter rise. When this occurs, record the cam timing off the wheel in degrees BTDC. Continue to rotate the engine through max lift (which you should also record) and continue to turn slowly until the dial indicator again reads 0.050 inch lift. Record this number as degrees after bottom dead center (ABDC). All we have to do is add the two numbers together plus 180 degrees (because we rotated the engine from BTDC to ABDC) and the sum will be intake duration at 0.050. Let's use the example of: 17 BTDC + 180 + 39 ABDC= 236 degrees, which means we have a cam with an intake lobe of 236 degrees of duration at 0.050. Use the same procedure for the exhaust side, which would be exhaust opening plus 180 degrees plus exhaust closing. As an example, exhaust opening will be 53 degrees before bottom dead center (BBDC) and exhaust closing will be 11 degrees ATDC for a total exhaust duration of 244 degrees. You could also measure the opening and closing points at 0.006-inch tappet lift if you wanted an "advertised" duration figure, but the 0.050-inch tappet lift numbers are more relevant. You can also easily compute valve lift by multiplying the maximum lobe lift by the rocker arm ratio. As an example, let's say the max lobe lift is 0.300 inch and our rocker ratio is 1.7:1. This makes it 0.300 x 1.7 = 0.510 inch.

If you want to know where the intake centerline falls, you can measure it by recording degree-wheel numbers 0.050 inch on either side of max lift, or you can do it mathematically. The first step for the math version is to divide the intake duration by 2 and then subtract the intake opening point. Using the above example, 236 degrees duration / 2 = 118 - 17 (intake opening) = 101 degrees intake centerline ATDC. The exhaust centerline uses a similar formula: duration / 2 minus exhaust closing. Again with the above example, we have a 244-degree exhaust lobe with an exhaust opening of 53 degrees BBDC and an exhaust closing of 11 degrees ATDC. This makes the formula: 244 / 2 = 122 - 11 = 111 degrees BTDC.

Once we know the centerlines of both the intake and exhaust lobes, we can compute the lobe-separation angle, which is merely intake centerline plus exhaust centerline divided by 2. In this case, we have 101 + 111 = 212 / 2 = 106 degrees, which means we have lots of overlap on this cam. Most lobe-separation angles are between 106 and 114, degrees although the stock cams used in the late-model LS engines are very wide at 117 degrees or more to improve idle stability. Hopefully this has helped you figure out the camshaft that's in your engine. Good luck.

Durable C4Chris Rabben; Las Vegas, NV: My question is regarding the C4 transmission in my '69 Mustang. It's time for a rebuild and I'd like to step out of my comfort zone and do the work myself, but I'm having a really hard time finding any books or information on doing a performance rebuild. It has to survive behind a 700hp nitrous small-block, so I need to put something pretty stout together. Are any full rebuild kits out there to handle this power level? I'm going to run a transbrake but just about all the brakes I find are for '70-and-newer C4s. Are these compatible with my '69 trans? I know the input shaft should be changed to the '70 26-spline, but what needs to be changed to swap that over?

Jeff Smith: This sounded like an intriguing question, Chris, so we immersed ourselves in the world of the racing C4. While 700 hp seems like a lot of steam for the little trans that was originally used only in front of small-block cars, the automatic transmission world has built some serious durability into these three-speed autos. The consensus opinion is that unless you have significant experience building performance automatics, the horsepower you propose almost dictates that you leave the buildup to one of the many C4-knowledgeable shops. That's why we're not going to go into a ton of detail on all the specific parts necessary to convert a C4. A shop relatively close to Las Vegas is Mike's Transmission in Lancaster, California. Mike's has a reputation for durable C4s and offers four different levels of C4 automatic starting with the Street/Strip version that goes for $950. The Competition version is $1,595, and the Ultimate C4 lists for $2,495.

For those of you on the East Coast, Dynamic Racing Transmissions (DRT) has earned an equally well-deserved reputation for building durable C4s. DRT also offers various levels of C4 including a trans DRT called the Mighty Mite that sounds like what you're going to need. Both companies recommend the hardened 26-spline input shafts, a solid forward drum, five- and six-clutch packs for the forward and direct drums, a large-diameter servo, and wide bushings. DRT also recommends what it calls a rollerized upgrade that exchanges roller bearings for the factory-style thrust washers and bushings. Roller bearings are capable of greater thrust loads and they also reduce friction, which not only reduces lost power but also lowers heat buildup in the transmission. DRT also has an external transbrake for these transmissions. Talk to Harold Miller at DRT and he can give you more specific advice for your application, including torque converter recommendations. Both of these companies can answer your specific questions concerning input shaft changes, transbrake applications, and other differences between the early and later C4 automatics.

Simple Battery Test
Electrical problems are often the bane of many car crafters. If your car sits for long periods of time even the minor electrical drain of a digital clock can be enough to drain a battery. For example, a typical stereo unswitched connection may pull only 100 milliamps (0.001 amp), but it's enough to radically discharge even a strong battery over a period of several months. That is why a small battery charge maintainer is a great idea to keep a charge in the battery, which also dramatically extends its life span.

8/8<strong>CC Quickies</strong><br />
In our This Guy's Garage department this month, Tom Owens says that when he takes his '57 Chevy to the dragstrip, he tows it with this cool '55 Chevy pickup that's painted to look like an old shop truck.

While major automotive shops can perform a battery load test to indicate battery condition, you can perform a similar test with nothing more than a digital multi-meter. The simplest form of this open circuit test is to merely connect the positive and negative leads from the multimeter to the battery with no load being applied. You can gain a relatively good sense of a battery's condition by its at-rest voltage. A slightly more sophisticated test is to put a 25-amp load across the battery for two minutes, such as a single large electric fan. Measure the voltage at the battery after the two-minute current draw and compare the voltage with the accompanying chart. If the voltage takes a serious tumble from its at-rest voltage after a two-minute current draw, you have an idea that the battery is nearing the end of its useful life. You'll note that there are slightly different voltage numbers for an absorbed glass matte battery like the Optima. Because these batteries offer less internal resistance, the voltage numbers are a little higher.